This invention relates to position detectors and, more particularly, to such detectors which produce a digital indication of the position of a movable mechanical member.
Many mechanical devices require the reliable measurement of the position of a movable element. For rotating elements such as the shafts of dynamoelectric machines, the angular position of the shafts can be determined through the use of commercially available shaft encoders. However, in certain applications such encoders are not acceptable. For example, position sensors required for the rotating shafts of brushless DC motors or AC starters for aircraft engines, must meet severe size and environmental requirements.
Magnetic types of position detectors are mechanically rugged. These types include permanent magnet voltage pickups or variable reluctance types of sensors. Voltage pickup types will not operate at very low speeds. Therefore, motor applications requiring operation at or near zero revolutions per minute must use the variable reluctance type of sensor.
Reluctance sensors typically use a toothed wheel which passes near an AC excited coil, varying the reluctance and therefore the inductance of the coil. The inductance change produces a detectable change in current or frequency of an oscillator which is resolved into a discrete logic level. A typical four bit sensor uses four toothed wheels and four coils to produce a digital word. The four bit word would provide a detector resolution of one part out of 16.
It is well known that linear and rotary encoders use so called Gray codes to avoid ambiguous readings as the bits change between digital words. Gray codes (which may have redundant bits) change only one bit at a time between adjacent positions. That is, adjacent words are identical except for one bit position.